Scoliosis is a spinal deformity mechanically characterized by torsion of the vertebral column, which affects several percent of the child population. The foremost deformity of scoliosis is axial rotation. This rotation in the transversal plane induces an automatic rotation of the vertebrae in the frontal plane which results in one or more frontal curvatures of the spine. Said curvatures may be structural at the level of the pathological zone or compensatory when they aim to reduce postural imbalance.
For each scoliotic curvature, particular vertebrae may be identified:                the apical vertebra (also called top vertebra of the curvature) is defined as being the vertebra the most lateralised and generally having the greatest axial rotation;        the limit vertebrae are situated at the upper and lower junctional limits; they are used to calculate the Cobb angle.        
The vertebrae of a curvature are deformed in the three planes in space. Wedging of the vertebrae is indicated by deformation of the vertebral body and loss of parallelism of the end plates. It is a three-dimensional phenomenon which modifies the orientation of the vertebral end plates due to an alteration of the process of vertebral growth. This deformation also affects the intervertebral discs, which can cause their premature degeneration.
Due to the close relationship between the thoracic vertebrae and the ribs, deformation of the spine leads to a modification of the orientation of the ribs and the shape of the thoracic cage. The phenomenon of torsion drags the ribs and creates a hump in the back of the patient, called gibbosity. Deformations of the thoracic cage may have a very significant impact on the respiratory function in the most severe forms of scoliosis.
Growth constitutes a determining factor in the constitution of a scoliosis. Indeed, scoliosis worsens during the period of rapid growth that peaks at puberty. Conversely, moderate scolioses remain stable after the end of growth.
There exists no curative treatment for the pathology from which scoliosis originates. The main objective of the treatment is thus to stop the aggravation of scoliosis either as a result of the residual growth potential or as a result of the structuring of compensation curvatures or the risk of secondary disc degeneration (osteoarthritis).
Conservative treatment by corset is at the present time the treatment of choice for scolioses that progress during growth. Most rigid corsets have the objective of correcting the translation of the apical vertebra in the frontal plane. They come under the principle of support—counter support. The application of a force on the apex of the main curvature makes it possible to translate the apical vertebra towards the median line. Counter supports are necessary; they are always iliac and thoracic.
The corset does not cure scoliosis because it does not make it possible to modify the deformation (the wedging) of each of the vertebrae. It acts like a supporting stake which makes it possible to avoid more serious aggravation. To be effective, the corset has to be worn 20 h/24 h until the child has stopped growing (sometimes more than 5 years). But the real effectiveness of corsets remains highly controversial. In addition, tolerance to the treatment and the psychological impact on the body image is important.
Surgical treatment is the last resort in the event of severe scoliosis. Its objective is the same as for the orthopaedic treatment, namely the stabilisation of the evolution of a scoliosis, but for more severe scolioses. Surgery consists in a correction of the deformation by means of screws, fasteners and rods and a fusion of the vertebrae together by means of a bone graft (vertebral arthrodesis). The objective of the surgical treatment is to fuse as few vertebrae together as possible in order to retain the maximum mobility in the vertebral column while restoring a satisfactory frontal and sagittal equilibrium. It is a weighty treatment associated with significant morbidity. However, conventional surgical treatment by arthrodesis cannot be envisaged for patients for whom the growth potential remains important.
Alternative methods have thus been described in animals and in humans. In humans, the technique used is convex epiphysiodesis by stapling. The idea is to block mechanically growth on the convex side of the curvature. The objective is to break the vicious circle by reducing the pressure on the concave side (corset effect) thanks to the maintaining of a correction of the stacking of the vertebrae but also to reduce wedging of the vertebrae by the effect of asymmetric blockage of growth. The technique used was initially direct stapling of the vertebrae together. However, such stapling was insufficient to correct really progressive scolioses.
Some authors have proposed using a dynamic spinal tethering device in place of staples [1]-[3]. This medical device, which is sold by the Zimmer Company under the name Dynesys™, is intended for anterior spinal tethering. This device comprises a bone anchoring screw and a synthetic ligament that connects the screws together. In publications [1]-[3], this device has been diverted from its normal indication and its effectiveness in the stabilisation or even the correction of scolioses has been evaluated. Although the results highlight the effectiveness of this device for the correction of scoliosis, several obstacles to the generalisation of its use for this indication remain. Firstly, the size of the screws is important. Indeed, once in place, the screws protrude by about 1 cm from the vertebra, which inevitably leads to rubbing on neighbouring organs, in particular the lungs. Secondly, putting in place the screws and the ligament is invasive, in so far as it requires incisions of the order of 2 to 3 cm. The result is considerable operational bleeding and a long operating time. Finally, the existing device does not make it possible to control the mechanical tension of the ligament, which is necessary to maintain the correction of the curvature.
U.S. Patent Publication No. 2013/0060287 describes a system comprising a flat ligament fixed by several vertebral implants along the vertebral column of a patient. This ligament is tensioned in the direction of its travel by means of a mechanical tensioning device which comprises, at the distal end thereof, a mechanism for driving the ligament which is actuated by a handle situated at the proximal end thereof. However, an important drawback of this device is that it has to be wedged against the vertebral implant so as not to slide on the vertebra, such that a high stress is applied to the vertebral implant during tensioning of the ligament, which is capable of tearing out or damaging the implant.